For vehicles that cannot steer (built to travel in a straight line) does having the wheels toe inwards help with straight line stability? That is, will it make the vehicle go straighter with slight construction asymmetries and frame warping?
I rather doubt it. Unless the resistance to drift for each tire is significantly asymmetric, i.e. each tire resists yaw to its side more than the other tire applies yaw to its opposite side, the car will still wander.
By way of comparison, take a look at train wheels. The train rides on two rails (duh), and the wheels' contact region is actually bevelled inwards. As a result, if the train drifts to the left, the left wheels rise slightly on the left rail, while the right wheels drop proportionately. . I don't see a way to do that with car wheels, since they are soft and have no rails to run up/down on.
Per the "edit2" below, I've removed the erroneous claim to gravity. Both referenced pages below clearly state that it's a matter of linear distance differences due to wheel radius at the contact point.
Good explanation and images at Wiki's Hunting Oscillation page.
EDIT: I've really addressed camber more than toe-in, but my impression is that overall the comments concerning straight-line tracking are still appropriate. If there's a greasemonkey who can correct me, please do.
EDIT2 It's not gravity! If the train drifts to the left, the left wheels contact the rail at an increased diameter so try to rotate more slowly, but their speed of rotation is locked to the right wheels (which are trying to speed up) by the axle, so the wheel set steers to the right. https://en.wikipedia.org/wiki/Wheelset_(rail_transport)